Sintered inorganic photoconductive elements and their method of preparation

ABSTRACT

PROCESS FOR PREPARING SINTERED PHOTOCONDUCTIVE ELEMENTS FROM MATERIAL WHICH CONTAINS II-VI GROUP COMPOUNDS AS A HOSE COMPONENT, IB GROUP ELEMENT AS AN ACTIVATOR AND VIIB OR IIIB GROUP ELEMENT AS A COACTIVATOR, WHICH MATERIAL IS SINTERED IN THE PRESENCE OF PREDETERMINED AMOUNTS OF VITREOUS ENAMEL AND SOLVENT FLUX. THE PROCESS PROVIDES PHOTOCONDUCTIVE ELEMENTS OF AN APPROPRIATE GEOMETRY AND THICKNESS AND WITH SATISFACTORY PHOTOSENSITIVITY AND STABLE PHOTOCONDUCTIVE PROPERTIES.

Nov. 28, 1972 SHIGEAKI NAKAMURA FI'AL SINTERED INORGANIC PHOTOCONDUCTIVE ELEMENTS AND THEIR METHOD OF PREPARATION Filed March 5, 1970 TIME SECONDS p m fi vOJ8765432 O @EQEOKQE kzmmmzo m m PZmEKDQOPOIQ QMNWZSEOZ 1 IO I02 I03 VOLTAGE VOLTS TIME DAYS INVENTOR smesm ummuan, mom uAKAMm HADAO KOHASHI BY 2 ATT RN United States Patent 3,704,265 SINTERED INORGANIC PHOTOCONDUCTIVE ELEMENTS AND THEIR METHOD OF PREPARATION Shigeaki Nakamura, Tadao Nakamura, and Tadao Kohashi, Osaka, Japan, assignors to Matsushita Electric Industrial Company, Limited, Osaka, Japan Filed Mar. 3, 1970, Ser. No. 16,134 Claims priority, application Japan, Mar. 7, 1969, 44/ 18,249 Int. Cl. C03g 5/00 US. Cl. 252501 6 Claims ABSTRACT OF THE DISCLOSURE Process for preparing sintered photoconductive elements from material which contains II-VI group compounds as a host component, Ib group element as an activator and VIIb or 1111) group element as a coactivator, which material is sintered in the presence of predetermined amounts of vitreous enamel and solvent flux.

The process provides photoconductive elements of an appropriate geometry and thickness and with satisfactory photosensitivity and stable photoconductive properties.

The present invention relates to a sintered photoconductive element and a process for making the same.

As is well known, photoconductive elements are manufactured from the Hb-VI group compounds, to which the 1b group element and VII or IIIb group element are added as an activator and as a coactivator, respectively. Such photoconductors are used in the form of single crystal, polycrystalline powder, powdered layer in an insulating binder, evaporated layer, sintered layer on a substrate and sintered moulded body.

Difiiculty is experienced in the production of reproducible single crystals or polycrystalline powders for practical reasons and, when such single crystals or polycrystalline powders are produced on a commercial basis, disproportionately high production cost is necessarily involved. This is the reason why such crystals or powders have failed to find practical applications in spite of their high photosensitivities. Where powdered layers are used as photoconductors in particular, the photoconductors are embedded homogeneously in an insulating binder, dispensing with any tactful techniques. These powdered layers are not accepted as entirely satisfactory for practical purposes, because the layers have a highly increased electrical resistance and excessive nonlinear current-voltage characteristics. It may also be mentioned that the layers are remarkably slow in responding to the intensity variations of an incident light.

Evaporated layers, sintered layers and sintered moulded bodies also have a practical and commercial significance. Nevertheless, the evaporation process as presently employed to the evaporated layers is concomitant with a drawback that the layers often contain one of the host materials with too great a stoichiometric ratio. These layers, therefore, should be subjected to an additional heat treatment for evaporating the excess amount of the host component. The photoconductive properties resulting from this heat treatment often differ remarkably from one layer to another when a large number of evaporated layers are fabricated.

The sintered layers and sintered moulded bodies have found a wide variety of applications because of their properties desirable in photoconductors. These layers and bodies are in close analogy with the single crystals in the photoconductive properties and exceed both of the powdered layers and the evaporated layers. Furthermore, the sintered layers and sintered moulded bodies are provided with such advantageous characteristics as high photosensitivity, quick response, production economy and linearity in current-voltage curves. With these characteristics, the sintered layers and bodies are widely applicable as photoconductive cells and photoelectric converter elements, which have made marked advances in practical applications.

Although the sintered layers and bodies can be used for various purposes, the geometry thereof should be determined depending upon the applications they are to be placed on. Of such geometry, it is the thickness that is the most important factor governing the photoconductive performance of the elements used. In some cases, it is required of a photoconductive element to have an increased thickness without detriment to the photoconductive properties and production reproducibility. This requirement can not be fulfilled to a satisfactory extent by the existing methods.

For example, in a conventional production method of sintered layers employing cadmium selenide as a host material in the form of fine powders and doped with copper and chlorine element as activators and coactivators, respectively, it is a usual practice to add a predetermined amount of cadmium chloride as solvent flux for dissolving the cadmium selenide. To prepare photoconductive elements from these substances, the substances are mixed with water to a uniform mixture paste, which, in some cases, are coated on a suitable substrate of glass, mica or ceramics by spraying, brushing or silk-screening. After drying, the mixture paste is sintered together with the carrier in a suitable atmosphere, for example, in an oven of a limited air circulation. According to this practice, crystallization of the host material during the sintering step invites recombination of the fine particles, thus causing an inevitable volume shrinkage of the processed photoconductive elements. For this reason, the relatively thick photoconductive layers manufactured according to the known practice tend to be dislocated from the carrier and to produce serious cracking therein. These drawbacks have not been overcome by such conventional practice.

In order to fabricate sintered photoconductive elements of relatively high thickness, a method proposed and utilized by which the sintered moulded bodies are manufactured. In this method, the material containing the host material, activator, coactivator and other additives, all of which are similar to those of the former example, are also mixed homogeneously and then pressed mechanically into a required form, for instance, to the form of pellet. When the moulding and drying operations are over, the mixture pellet is sintered to promote crystallization of the host material and to introduce the impurities such as the activator and coactivator into the host crystal. According to this prior art method, relatively thick elements can be fabricated with ease. The geometry thereof are also restricted in a narrow range with the resultant limitation in the production reproducibility. In particular, it is found that the photoconductive elements having relatively wide operating area or adhered directly to the substrate are affected more seriously than the sintered layer by the cracking or dislocation.

It is therefore a primary object of the present invention to provide a process for preparing sintered elements by adding predetermined amounts of vitreous enamel powders and solvent flux to a host material to be sintered.

It is another object of the invention to provide a process for preparing sintered photoconductive elements from material which contains IIb-VI group compounds as a host component, Ib group element as an activator and VII or IIIb group element as a coactivator to be incorporated in the crystal of the host material, this material being sintered, in the presence of predetermined amounts of vitreous enamel and solvent flux.

It is still another object of the invention to provide a process for preparing improved photoconductive elements of relatively great thickness, which elements have advantageous photoconductive properties such as reduced aging effect on the photocurrent thereof, stabilized characteristics of the dark current thereof with an extremely high voltage applied, freedom from time-drift of the dark current, linearity in current-voltage curve and reproducibility of the dark current for various operating conditions.

It is a further object of the invention to provide a process for preparing sintered photoconductive elements with increased production reproducibility in their geometry and with sufficient allowance of the concentrations of the added impurities.

Advantages and features of the invention will be apparent to those skilled in the art from the following description, taken in conjunction with accompanying drawing, in which:

FIG. 1 is a perspective view sketchily showing a sintered photoconductive element fabricated by the method according to the invention;

FIG. 2 is a graphical representation both of photocurrent and dark current plotted against the applied voltage;

FIG. 3 is a plot illustrating typical time-drift characteristics of the dark current; and

FIG. 4 is a plot illustrating aging effects on the photocurrent.

Prior to entering into the detailed discussion of the examples and figures, brief explanation will be given in order to make understood the basic concept of the production method according to the invention.

A method proposed by this invention is characterized in that the host material doped with activators and coactivators are sintered in the presence of predetermined amounts of vitreous enamel powders and solvent flux. In the discussion to follow, the method is exemplified as applied for fabricating a sintered photoconductive ele ment, by way of example only.

The host materials may be selected from the II-VI group compounds, such as for example, cadmium sulphoselenide, cadmium sulphide and/or cadmium selenide. Predetermined amounts of activators and coactivators, the former being selected from [b group elements (copper, silver and the like) and the latter being selected from VIIb group elements (chlorine, bromine, iodine and the like), are added to the host materials.

In one preferred method of the invention, the host material is mixed with the activator by the addition of distilled water. After drying is over, this mixture is fired to promote crystallization a part of the host material and to incorporate the activator into the host material. The fired mixture is then pulverized into fine particles. These fine particles are homogenized into a uniform mixture paste by the addition of a suitable dispersing agent. This mixture paste, after being coated on a heat resistant and chemically inert substrate so as to form a layer, is dried and sintered together with the substrate in the presence of predetermined amounts of vitreous enamel powders and solvent flux. Nearly all the amount of the coactivator is incorporated in the host material by the solvent flux during the sintering step.

The added solvent flux melts during the sintering step, serving partly as a solvent for the host material, partly as a supplier of the coactivator and partly as an agent for aiding in the crystallization of the host material. This solvent flux is evaporated into the surrounding atmosphere at the end of the sintering step. On the other hand, the employed enamel powders are incorporated into the porous crystal of the host material during the sintering step after they have softened. These enamel powders, therefore, act as an agent for reducing the variation in volume without detriment to crystallization of the finished photoconductive layer.

In another preferred method of the invention, firing may be conducted twice or more, if desired. For such cases, a variety of variations may be conceived; for example, addition of the activator and coactivator may be performed before the different firing steps. However, it should be noted that sintering takes place at one step. In still another preferred method of the invention, the starting material containing the host material, activator, coactivator, vitreous enamel powders and solvent flux is mixed altogether by the addition of distilled water. The thus obtained mixture is dried and then pulverized into fine particles. These particles are homogenized with the aid of the dispersing agent, to form a mixture paste. This mixture paste is then coated on the substrate in the form of layer and dried. After drying is over, this layer is sintered together with the substrate so as to promote crystallization of the host material and to incorporate the activator as well as coactivator into the host material.

In the following discussion, photoconductive properties of the elements of the invention will be compared with those fabricated according to the conventional methods in conjunction with the figures of the accompanying drawing.

Now referring to FIG. 1, there is shown a photoconductive element fabricated by the method according to the present invention. The photoconductive element, which is designated generally by 10 is provided with a photoconductive layer 11, substrate plate 12 on which the photoconductive layer 11 is coated and a pair of metal electrodes of aluminum vacuum-evaporated on the photoconductive layer 11. In the experiments conducted by us, a source of illumination (not shown) is switched on to measure the photocurrent of the element 10, when a predetermined voltage is applied between the tWo electrodes 13. A suitable amount of electric current i.e. the photocurrent is passed through that area of the photoconductive layer '11 which is intervened between the electrodes 13. The applied voltage may be either a DC. or A.C. voltage but, for the simplicity of discussion, the same is described as a DC. voltage in the following figures.

Referring to FIG. 2, there is shown current-voltage characteristics of the photocurrent and the dark current indicated by 14 and 15, respectively, of a typical photoconductive element of cadmium selenide type, which element is fabricated by the method according to the present invention and will be referred to as element A. Curves 16 and 17 represent photocurrent and dark current, respectvely, of a conventional photoconductive element of cadmium selenide type, which element will be referred to as element B. This element B is placed under the same experimental conditions as the element A of the invention. The dark currents 15 and 17 are measured two minutes later after the illumination source is switched off. The photocurrents 14 and 16 are measured when an incandescent light of 10 luxes is incident on the surface of the elements A and B from the illumination source of 2854" K. As is seen in FIG. 2, the absolute value of the photocurrent 14 is higher than that of the photosensitivity to the element B. Moreover, it is also found that the dark current curve 15 is always located below the curve 17 in the experimental range. This means that the dark characteristics of the element of the invention is superior to that of the conventional one. It should be appreciated as a favourable feature of the invention that the characteristic curve 15 is substantially linear even in the high voltage region. In other words, the photoconductive element according to the present invention obeys approximately the Ohms law and therefore lends itself to a wide variety of applications.

, FIG. 3 shows a series of transitional phenomena of the dak current of the photoconductive elements with a constant voltage applied. Experiments conducted are divided into three stages, the first stage of timedrift of the dark current, second stage of restoration of the dark current after the source of illumination has been switched off and third stage of reproducibility of the dark current when the elements are again energized. In the measurements, the photoconductive elements are used which are similar to those A and B described in connection with FIG. 2, but the applied voltage is kept constant at 100 volts.

In operation, the voltage of 100 volts is impressed on the elements A and B at time T During the first stage of 160 seconds from the time T the dark currents of the elements A and B behave in a manner represented as curves 18 and 19, respectively. As shown, the dark current of the element B drifts considerably, whereas that of the element A once decreases and is thereafter maintained at a constant value. At time T which is 160 seconds later than the time T the illumination source irradiating luxes illumination is turned on. After this illumination source is turned off at time T i.e. during the second stage, the dark current of the element A shown as curve 21 is soon restored to the initial constant value. To the contrary, the dark current of the element B shown as curve 22 remains at an elevated value until it drops suddenly to zero and its absolute value exceeds that of the element A during the second stage. At time T when the applied voltage is cut off, the dark currents passing through the elements A and B are then reduced to zero. At time T, which is 60 seconds later than the time T the voltage of 100 volts is again impressed on the elements A and B. During the third stage after the time T the dark current of the element A shown as curve 23 once increases instantaneously and thereafter decreases to restore the initial constant value, whereas that of the element B shown as curve 24 do not behave similarly to the first stage.

It therefore follows that the photoconductive properties of the sintered photoconductive element according to the invention exceeds those attainable by the conventional element. More specifically, the element of the invention has sufiicient stability in its dark current When a constant voltage is applied thereto. Satisfactory reproducibility is also attained by the element of the invention for the operations where the source of illumination is terminated, where the applied voltage is cut off and where the particular voltage is again impressed on the particular elements Without any voltage applied previously.

In FIG. 4 showing the aging effects on the photocurrents of the elements A and B, curves 25 and 26 correspond to the photocurrent characteristics of the elements A and B, respectively. The two elements are subjected to aging by being packed in respective envelopes evacuated to the order of 10- 1() torr. For measurement of the photocurrent, the aged elements A and B are unpacked and exposed to open air one by one. In this measurement, the applied voltage is 100 volts and the irradiated illumination is 10 luxes. As is observed in FIG. 4, the photocurrent of the element A is less impaired by aging elfects than that of the element B.

According to the method of the invention, sintered elements of desired dimensions and shapes and especially of relatively high thickness can be prepared with ease and satisfactory production reproducibility. Such elements can be adhered so firmly to the surface of a substrate as not to be dislocated or cracked. Where the method is employed for fabricating sintered photoconductive elements, various beneficial photoconductive properties can be obtained. Thus, the photoconductive elements fabricated by this method will be noted for their linearity of the dark current with a considerably high voltage applied, reduced time-drift of the dark current, satisfactory reproducibility of the dark current for transitional stages of different operations and reduced aging effects on their photocurrent.

The method according to the present invention, therefore, can provide photoconductive elements of an appro- -o1oqd AJOIOEJSIJES qn/n pua ssauxa tp, pun Ansuroafi e npd sensitivity and stable photoconductive properties. Thus, the method and photoconductive elements of the present invention can find a wide variety of practical applications, especially for photoelectric converters or image devices. It should, however, be borne in mind that application of this method are not limited to sintered photo conductive elements using the II-VI group compounds as the host material. It will be accepted that other sintered semiconductors and the like with or without impurities can also be fabricated by the method according to the present invention.

In order that the present invention may be more fully understood, the following examples are given by way of illustration only.

EXAMPLE 1 The starting material consists of g. of 99.999% cadmium selenide and 2 cc. of 0.1 mole aqueous solution of divalent copper chloride. By the addition of 200 cc. of distilled water, the starting material is dispersed to obtain a homogenized mixture. Then, this mixture is dried by heating to a sufficient temperature to evaporate moisture therefrom. The throughly dried mixture is fired in an atmosphere of a limited air circulation, for example, in an oven of fused quartz tube which is heated to 600 C. for about an hour. At the end of this heat treatment, the oven is opened and the fired mixture is left to cool oif. After cooling, this mixture is taken off from the oven and then pulverized into fine particles.

The thus obtained 100 g. of fine particles are added with 15 g. of cadmium chloride as a solvent flux, 5 g. of vitreous enamel powders and 200 cc. of distilled water are added. These substances are mixed and the resultant mixture is then dried to evaporate the remaining Water. This dried mixture is again pulverized into fine particles with use of a suitable mortar of agate, for instance. An organic dispersing agent such as octylalcohol is added to the fine particles, which are then agitated sufficiently until a mixture paste of an appropriate consistency is obtained. This mixture paste of desired thickness is coated on a glass plate acting as a substrate by silk-screening or by the use of a spacer and is thereafter dried to evaporate the dispersing agent. This glass plate having thereon dried powdered layer is placed in an oven of a limited air circulation and is sintered therein. These sintering conditions are determined depending upon the thickness of the powdered layer. In this example, the layer of 300 micron thickness is used, which is sintered in the oven maintained at 600 C. for 20 to 30 minutes and then cooled off. As a result of this sintering step, the layer is activated to become photosensitive.

During the sintering step, the cadmium chloride acting as a solvent flux melts to dissolve a part of the host material and to promote crystallization of the material and evaporates completely into the open air. Before evaporating, chlorine originated from cadmium chloride is introduced as a coactivator into the crystal of the host material. The applied vitreous enamel softens during this sintering step and is incorporated into the porous host component, thus reducing the volume variation of the layer without impairing crystallization of the host component. As a result, the host component incorporates the vitreous enamel thereinto and therefore forms a continuous, stable polycrystalline layer which is adhered firmly to the glass plate. The thus obtained photoconductive layer can, if desired, be provided with a pair of electrodes of aluminum, indium, gold or silver. These electrodes may be mounted on the surface of the photoconductive layer by any method such as vacuum-evaporation, coating or spraying.

The vitreous enamel used in this example contains, by weight percentages, silicon dioxide 17%, diboron trioxide 28%, zinc oxide 23%, barium oxide 14%, sodium oxide 14% and potassium oxide 4%. Vitreous enamels as applicable in this invention are not limited to that of the above-described components. According to our experiments, enamels of borosilicate type which have a softening point lower than or equal to the sintering temperature of the host components can also be utilized. It is found by our experiments that vitreous enamels of borosilicate type are most preferable because they provide photoconductive elements of advantageous properties. Such borosilicate enamels can be employed as containing, by weight percentages silicon dioxide 14.5 to 44.1%, diboron trioxide 23.7 to 28.7%, zinc oxide 2.2 to 23.5%, barium oxide to 14.6%, sodium oxide 10.9 to 15.4%, potassium oxide 0 to 4.2%, titanium oxide 0 to 9.0%, aluminum oxide 0 to 2.7%, and calsium oxide, magnesium oxide, ferric oxide and/or lead oxide 0 to 1.2%, respectively, with their softening points 450 to 515 C. and their volume expansion coeflicients 260 to 340 10-"/ C. protect the flasks vapor space from radiated heat.

The amount of the vitreous enamels to be introduced into the host component can be determined relatively free- 1y depending upon only the particular design requirements. This amount will be hereinafter defined as volume percentage 6. Throughout the examples, as 6 increases from 0%, photosensitivity of the photoconductive element obtained also increase gradually and attains its maximum value for 6 of 7 to 8%. As 6 exceeds the value 7 to 8%, the sensitivity decreases gradually due to the high resistivity of the vitreous enamel incorporated in the porous host component. This is because the photocurrent of the element is gradually prohibited from passing through the element with the increasing resistivity of the element. When reaches 16%, the photosensitivity of the element reaches the same level 'as that of the element which is void of the enamel.

It is therefore desired that 6 is determined to lie in the range from up to 16% from the practical view point so as to assure suificiently high photosensitivity. On the other hand, it is also found by our experiments that the mechanical strength of the photoconductive element prepared is insuflicient for practical applications where 6 is less than 5%. Consequently, to obtain a photoconductive element with sufficient mechanical strength and high photosensitivity, it is desired that 6 be in the range of 5 to 16%.

With respect to a solvent flux, cadmium chloride is employed in this example for dissolving the host component. Such solvent fluxes of compound type as the compounds of cadmium bromide and cadmium iodide are also applicable for the same purposes.

Dispersing agents as applicable in this example are required to have a suitable consistency and preferably not to dissolve the solvent flux. Such dispersing agents are not limited to octylalcohol used in this example. Other agents, which can be used, are alcohols having more carbon atoms than amyl alcohol and glycols having more carbon atoms than ethylene glycol. Water may be used for the same purpose, if desired.

In this example, soda-lime glass is also used as a substrate for supporting thereon the photoconductive layer.

This soda-lime glass is advantageous in that it has softening point of about 690 C., thermal expansion coefiicient of 310x 10- C. and high electric resistivity. Substrate are, however, not limited to such soda-lime glass inasmuch as they are heat-resistant, chemically inert and have substantially the same volume expansion coeflicient as that of the vitreous enamel. Substances as applicable as such substrate are, for example, other glasses such as potash glass, fused quartz, heat resistant glasses as known under the trade name of Pyrex glass fabricated by Corning Glass Works Co., mica, rock crystal, ceramic and metal materials such as molybdenum. As is described in the above, these substances are characterized in that they are chemically inert both to the solvent flux and to impurities, in that they are heat-resistant to a temperature higher than 600 C. which corresponds to the sintering temperature, i.e. they are provided with softening or melting points higher than the sintering temperature, and in that they have substantially the same thermal expansion coefficients as that of the vitreous enamel added.

EXAMPLE 2 The starting material consists of g. of 99.999% cadmium selenide, 2 cc. of 0.1 mole aqueous solution of divalent copper chloride and 5 g. of cadmium chloride. In this example, the solvent flux or cadmium chloride is added to the host component prior to the firing step. By the addition of 200 cc. of distilled Water, the starting material is dispersed to obtain a homogenized mixture. This mixture is dried by heating to a sufficient temperature, for example, C. The dried mixture is fired in an oven of a limited air circulation at 600 C. for about an hour. After cooling off, this fired mixture is pulverized into fine particles. The succeeding steps are similar to Example 1.

The method of Example 2 is characterized in that cad mium chloride as the solvent flux is also added before the firing step. As a result, during the firing step, chlorine is introduced as a coactivator into the host component together with copper as an activator.

EXAMPLE 3 99.999% cadmium selenide employed as host component in Examples 1 and 2 is replaced by cadmium sulphide in this example. The steps for fabricating therefrom a photoconductive element are similar to those described in Example 1.

EXAMPLE 3' Materials used are the same as those of Example 3. The steps are, however, similar to those described in Example 2.

EXAMPLE 4 A mixture of 99.999% purity of cadmium sulphide and cadmium selenide is used as a host component in place of cadmium selenide of Examples 1 and 2. The steps are similar to those of Example 1.

EXAMPLE 4' Materials used are the same as those of Example 4. The steps are, however, similar to those described in Example 2.

EXAMPLE 5 99.999% cadmium sulphoselenide is employed as a host component in this example and the steps are similar to those of Example 1.

EXAMPLE 5' Materials used are the same as those of Example 5. The steps are, however, similar to those described in Example 2.

EXAMPLE 6 Using host components as described in Examples 1 to 5', the mixture paste is pressed or moulded onto the carrier plate to be formed into a layer of a desired thickness and shape. Other steps excepting the coating step are similar to those of Example 1.

EXAMPLE 6' Materials used are the same as those of Example 6. The steps are, however, similar to those described in Example 2.

Alternatively, similar photoconductive elements may be fabricated by a method of a single sintering step, in which no firing is performed. In this simplified method, the material to be used contains the host component, activator and coactivator elements, vitreous enamel, solvent flux and so on. This starting material is mixed altogether and coated or moulded on a substrate before the single sintering step. Therefore, it can be said that the crystallization of the host component and the introduction of the activator and coactivator into the host component are performed during this sintering step. In order to prepare highly sensitive photoconductive elements, however, more care should be taken of the sintering conditions than those prepared by the methods explained in Examples 1 to 6'. A typical experiment conducted by us will be explained in connection with Example 7.

EXAMPLE 7 The starting material consists of 100 g. of 99.999%

cadmium selenide, 2 cc. of 0.1 mole aqueous solution of divalent copper chloride, 15 g. of cadmium chloride as solvent flux and g. of vitreous enamel powder. By the addition of 200 cc. of distilled water, this starting material is dispersed to obtain a homogenized mixture and is then dried thoroughly. After this drying step, the dried mixture is pulverized into fine particles by the use of a suitable mortar of agate. These particles are then formed into a mixture paste by the addition of a predetermined amount of an organic dispersing agent. This mixture paste is coated or moulded on a substrate to form a layer of a desired thickness by any of the methods described before. After being dried to remove therefrom the dispersing agent, this layer is sintered together with the substrate in an atmosphere of a limited air circulation at 600 C. for 30 to 40 minutes.

What is claimed is:

1. A process for preparing a sintered photoconductive element comprising:

a host component selected from the group consisting of cadmium selenide, cadmium sulphide and cadmium sulphoselenide;

an activator selected from the group consisting of copper and silver;

a coactivator selected from the group consisting of chlorine, bromine and iodine; and

about 5% to about 16% by volume of vitreous enamel of borosilicate type containing, by weight percentage, silicon dioxide of about 14.5 to about 44.1%, diboron trioxide of about 23.7 to 28.7%, zinc oxide of about 2.2 to about 23.5%, barium oxide of up to about 14.6%, sodium oxide of about 10.9 to about 15.4%, potassium oxide of up to about 4.2%, titanium oxide of up to about 9.0%, aluminium oxide of up to about 2.7%, calcium oxide of up to about 1.2%, magnesium oxide of up to about 1.2%, ferric oxide of up to about 1.2%, and lead oxide of up to about 1.2%; said process comprises: mixing said host component and said activator; firing the mixture at about SOD-700 C.; pulverizing the fired mixture into fine particles; admixing said coactivator, said vitreous enamel, solvent flux of about 5 parts to about 20 parts, by weight, per 100 parts of said host component, and a dispersing agent with said fine particles; agitating the admixture to form a mixture paste; drying said admixture paste to evaporate said dispersing agent; sintering said admixture at about 500-700" C.; wherein said solvent flux is selected from the group consisting of cadmium chloride, cadmium bromide and cadmium iodide, and said dispersing agent is selected from the group consisting of alcohols having more carbon atoms than amyl alcohol, glycols having more carbon atoms than ethylene glycol, and water.

2. A process for preparing a sintered photoconductive element comprising:

a host component selected from the group consisting of cadmium selenide, cadmium sulphide and cadmium sulphoselenide;

an activator selected from the group consisting of copper and silver;

a coactivator selected from the group consisting of chlorine, bromine and iodine; and

about 5% to about 16% by volume of vitreous enamel of borosilicate type containing, by weight percentage, silicon dioxide of about 14.5 to about 44.1%, diboron trioxide of about 23.7 to about 28.7%, zinc oxide of about 2.2 to about 23.5%, barium oxide of up to about 14.6%, sodium oxide of about 10.9 to about 15.4%, potassium oxide of up to about 4.2%, titanium oxide of up to about 9.0%, aluminium oxide of up to about 2.7 calcium oxide of up to about 1.2%, magnesium oxide of up to about 1.2%, ferric oxide of up to about 1.2%, and lead oxide of up to about 1.2%; said process comprises: mixing said host component, said activator and said coactivator; firing the mixture at about 500-700 C.; pulverizing the fired mixture into fine particles; admixing said coactivator, said vitreous enamel, solvent flux of about 5 parts to about 20 parts by weight, per parts of said host component, and a dispersing agent with said fine particles; agitating the admixture to form a mixture paste; drying said admixture paste to evaporate said dispersing agent; sintering said admixture at about 500-700 C.; wherein said solvent flux is selected from the group consisting of cadmium chloride, cadmium bromide and cadmium iodide, and said dispersing agent is selected from the group consisting of alcohols having more carbon atoms than amyl alcohol, glycols having more carbon atoms than ethylene glycol, and water.

3. A process for preparing a sintered photoconductive element comprising:

a host component selected from the group consisting of cadmium selenide, cadmium sulphide and cadmium sulphoselenide;

an activator selected from the group consisting of copper and silver;

a coactivator selected from the group consisting of chlorine, bromine and iodine; and

about 5% to about 16% by volume of vitreous enamel of borosilicate type containing, by weight percentage, silicon dioxide of about 14.5 to about 44.1%, diboron trioxide of about 23.7 to 28.7%, zinc oxide of about 2.2 to about 23.5%, barium oxide of up to about 14.6%, sodium oxide of about 10.9 to about 15.4%, potassium oxide of up to about 4.2%, titanium oxide of up to about 9.0%, aluminium oxide of up to about 2.7%, calcium oxide of up to about 1.2%, magnesium oxide of up to about 1.2%, ferric oxide of up to about 1.2%, and lead oxide of up to about 1.2%; said process comprises: mixing said host component, said activator, said coactivator, said vitreous enamel and solvent flux of about 5 parts to about 20 parts by weight, per 100 parts of said host component; pulverizing the mixture into fine particles; admixing a dispersing agent to the pulverized mixture; agitating the admixture to form a mixture paste; drying said mixture paste to evaporate said dispersing agent; sintering said admixture 11 at about 500-700 (3.; wherein said solvent flux is R f r n Cit d selected from the group consisting of cadmium chlo- UNITED STATES PATENTS ride, cadmium bromide and cadmium iodide, and said dispersing agent is selected from the group con- 2,937,353 5/1960 wsserman 33315 sisting of alcohols having more carbon atoms than 5 2,908,594 10/1959 117201 amyl alcohol, glycols having more carbon atoms 3,170,885 2/1965 MOTPSOB at than ethylene glycol, and water. 3,324,299 5/1967 Schull 250-211 4. A sintered photoconductive element prepared by the process set forth in claim 1. GEORGE F. LESMES, Primary Examiner 5. A sintered photoconductive element prepared by 10 M. B. WITIENBERG, Assistant 'Examiner the process set forth in claim 2. U S Cl X R 6. A sintered photoconductive element prepared by 117 201 the process set forth in claim 3. 

